Conical scan monopulse radar system



Dec. 19, 1961 R. M. ASHBY ETAL 3,014,214

comm. SCAN MONOPULSE RADAR SYSTEM Filed June 27, 19s? 5 Sheets-Sheet 1INVENTORS. ROBERT M. ASHBY JAMES A MARSH ATTORNEY 5 Sheets-Sheet 2INVENTORY. ROBERT M. ASHBY ATTORNEY Dec. 19, 1961 R. M. ASHBY ETALCONICAL. SCAN MONOPULSE RADAR SYSTEM Filed June 27, 1957 n w Al 5 13828523: 5E J mm M Zn on fidaoo E m? l w w mommm 5 I l l l J 5x5 0 on; Non5 $24 58 02:. FOE mu 1 mm vm ll\ mm r I I L :52 3 wzifiom u 15138 I .6 6mm muEim w. mew mv l uni. mat x8 nv\ mmiaoo nrwwm $/,5C n m J wms i oz;528 Nv E3058 Q 6 5 v mew NN H H moEHbm Non 5m 3 E 23 55m 55: I mnzm m0Zm Dec. 19, 1961 R. M. ASHBY ETAL 3,014,214

CONICAL SCAN MONOPULSE RADAR SYSTEM Filed June 27, 1957 5 Sheets-Sheet 3I5\ 23 47 FEED B RANGE CHANNEL L SIDE\ SLOT COUPLER ERROR CHANNEL 0 FEEDA l 90 Eb ER {$EOI E E P -O I80 F o E E02 E5 90 FIG. 4

FEED B '5 RANGE CHANNEL I SIDE SLOT CHANNEL (zglg' LER M, FEED A .IBO

7| E -JI I Eu .1: E -II FIG. 5

INVENTORS. ROBERT M. ASHBY JAMES A. MARSH ATTORNEY Dec. 19, 1961 ASHBYETAL I 3,014,214

CONICAL SCAN MONOPULSE RADAR SYSTEM Filed June 27, 1957 5 Sheets-Sheet 4FEED A FEED B FIG. 6

PARABOLOID 2o AZIMUTH FEED HORN A AXIS AZIMUTH AXIS ELEVATION E I AxisFEED HORN a 7 ELEVATION AXIS l9 FIG. 9

INVENTORS.

ROBERT M. ASHBY JAMES A. MARSH ATTORNEY De -19,1961 R. M. ASHBY Em.3,014, 1

CONICAL SCAN MONOPULSE RADAR SYSTEM Filed June 2'7. 195'? 5 Sheets-Sheet5 INVENTORS. R0 T M. ASHBY By JAM A MARSH ,aaaw

ATTORN EY United States Patent Inc.

Filed June 27, 1957, Ser. No. 668,431 Claims. (Cl. 343-41) Thisinvention relates to radar systems.

Highly directional radar antennas have been developed in the past whichconcentrate a substantial part of the transmitted radiant energy from aradar in a very small, highly directional beam. These antennas areusually used to obtain the range and bearing indications of a reflectingtarget with respect to the position of the transmitter-receiver. Thetransmitter and antenna direct pulses of radiant energy at the target.The radar set measures range by recording the time lapse between thetransmission pulse and the reception of the echo. Conventional radarsets measure bearing by determining the angular position of the antennaat the time the echo is received.

Because of the highly directional characteristic of the transmitting andreceiving antenna only those targets which are within a very small solidangle are detected by the radar set. Since the sensitive axis of theantenna must first be aligned with the line of sight to the targetbefore any echo can be received, various scanning systems have beendeveloped in the past to program scan the sensitive axis of the antennaover a large angular area in order to search. Most of these scanningsystems involve the physical rotation of the reflector element of theantenna or antenna feed system about an axis. The physical rotation witha high degree of accuracy of such a reflector element, or feed, requiresspecial balancing and torquing devices.

It is accordingly one of the objects of this invention to provide ascanning system without spinning antenna reflector, or feed, elements.

In my co-pending application, Serial No. 607,516, filed August 31, 1956,I have described a conical scanning system employing a four-hornemitter. My present object is to accomplish conical scanning from atwo-horn feed bridge.

Further objects of the invention are to accomplish conical scantransmission in an improved manner as well as permitting transmission orreception of error track monopulse signals and reception of conicalrange signals.

Other and further objects, features and advantages of the invention willbecome apparent as the description proceeds.

In carrying out the invention in accordance with a preferred formthereof, a microwave transmitter is provided capable of generatingenergy of a frequency transmissible through waveguides. Two waveguidechannels from the transmitter are provided, one feeding a range or sumchannel and the other a direction or error track channel. An antenna ofthe parabolic reflector type is employed, arranged for illumination by atwo-horn feed bridge with provisions for extracting tracking errorsignals in two orthogonal planes, that is azimuth and elevation, as wellas extracting range signals for monopulse radar operation.

Patented Dec. 19, 1961 horns are located on a line which makes an angleof 45 with both the elevation and azimuth axes. The two feed horns areof the J type for excitation by a center rear feed technique withwaveguides tending to isolate one horn from the other, so that one hornilluminates the portion of the reflector to the left of the waveguides,whereas the other horn illuminates the portion of the reflector to theright of the waveguides. The isolation provided by these waveguides issuflicient to cause a fairly large separation in terms of a wave lengthof the centers of phase of the two halves of the antenna, each more orless separately excited by one of the two feed horns.

A duplexer is interposed in the range channel consisting of a pair ofside slot couplers with a pair of receiver protection tubes commonlyknown as transmit-receive or TR tubes interposed between the two sideslot couplers. A range receiver is connected to the signal outlet portof the duplexer. A similar duplexer with an error receiver connected tothe signal outlet port is interposed in the error channel. In addition,a continuous phase shifter is interposed in the error channel betweenthe duplexer and the transmitter.

The proper combination of transmitted signal phase and amplitudes in therange and error channels produces conical scanning of the radiatedenergy.

The duplexers prevent the direct passage of energy from the transmitterto the receivers while permitting reflected energy received by theantenna to be directed to the range and error receivers in accordancewith the monopulse system instead of the conical scan system.

This system accordingly provides a conical scan radiated antenna beamfor locating a target. The same apparatus serves also for tracking thetarget by the highly precise monopulse system.

A better understanding of the invention will be afforded by thefollowing detailed description considered in conjunction with theaccompanying drawings, in which FIG. 1 is a plan view of a two horn feeddevice mounted in a fragmentarily shown reflector for a radar trackingsystem;

FIG. 2 is a cross-sectional view of the device of FIG. 1 represented ascut by a plane 2-2, as seen looking in the direction of the arrowsindicated on FIG. 1;

FIG. 3 is a block diagram of the microwave portions of the radar systememploying the two-horn feed device of FIG. 1;

FIG. 4 is a diagram illustrating the principle of operation by whichvarious feed excitations are separated into range and error channels;

FIG. 5 is a corresponding diagram illustrating the conditions with adifferent phase relationship;

FIG. 6 is a diagram illustrating the phase relationship between theenergy in the feed horns at successive instants of time within a cycleof phase variation during conical scan;

FIG. 7 is a diagram representing the effective phase center relationshipbetween the energy illuminating the parabolic reflector;

FIG. 8 is a schematic diagram representing the positional relationshipbetween the waveguides and the horns of the apparatus of FIGS. 1 and 2;

FIG. 9 is a polar graph illustrating the distribution of transmittedenergy in the range channel for two times during the conical scan cycle;and

FIG. is a polar graph illustrating the monopulse range and errorreception patterns.

Like reference characters are utilized throughout the drawing todesignate like parts.

As illustrated in FIG. 3, a system is employed comprising aschematically represented parabolic reflector 11 having a center opening12 (FIG. :1) in which the base '14 of a two-horn feed device 13 ismounted for illuminating the reflector 11. As shown more clearly in FIG.2, the two-horn feed device 13 comprises a pair of adjacent, rectangularcross section, center waveguides and 16 secured to the base 14 forelectrical connection to waveguides (not shown in FIG. 1) representedschematically by lines 16 and 17 in FIG. 3. The ends of the waveguides15 and 16 are turned in J form with elbows 17 and 18, the open ends ofwhich form feed horns 19 and 20. As illustrated in FIG. 8, the feedhorns 19 and 20 are located with their centers on a line which makes anangle of 45 with both the elevation and azimuth axes and the arrangementis such that the horns are in the vicinity of the focal point of theparabolic reflector 1.1.

In order to allow for separation of range and error informationcontained in the energy received through the waveguides 15 and 16, a 90phase shifter 22 is provided which actually consists of additionallength of waveguide in the elbow 17 as illustrated in FIG. 1. Forseparating received range and error-track energy in monopulse operation,a hybrid bridge 23 is interposed in the waveguides 15 and 16 and a pairof duplexers is provided. The hybrid bridge 23 may take the form of aside slot coupler. One duplexer comprises a dual TR tube 24 and two sideslot couplers 25 and 31, whereas the other duplexer comprises a dual TRtube 26 and two side slot couplers 27 and 32. For employing thesuperheterodyne principle of reception, a local oscillator 28 isprovided with a range mixer 29 and an error track mixer 30 having inputchannels from the local oscillator 28 and also receiving inputs from theduplexer through side slot couplers 31 and 32.

For producing the transmitted wave for use in the radar system asuitable oscillator such as a magnetron 34 is provided. From themagnetron 34 there is channel 35 through a test coupler 36, an automaticfrequency control coupler 37, conventional azimuth and elevationrotating joints 38 and 39, and a waveguide into a power splitter 44.

The power splitter 44 may be of the side slot coupler type. It may bealso one of the types described in volume IX of the MassachusettsInstitute of Technology Radiation Laboratory Series: MicrowaveTransmission Circuits, Sections 8.19, 8.20 and 8.22 on pages 522-528,533536 under the headings Waveguide Power Dividers and Switches. Thepower splitter 44 has two output channels 42 and 43, in one of which aFox type phase shifter 46, is introduced. Channels 42 and 43 supplyenergy to range and error channel waveguides 47 and 48. The powersplitter 44 and phase shifter 46 control amplitude and phase relationsof the energy in the range and error channel waveguides 47 and 48 toproduce conical scan transmitted radiation. By controlling the powersplitter 44, all of the energy may be transmitted in the range channel47 as in a conventional monopulse system. The continuous phase shifter46 may be of the Fox type comprising a rotatable section of a roundwaveguide containing a 180 phase shift delta section. The phase shifter46 may, for example, be of the type described in the copendingapplication of Robert M. Ashby, Serial No. 606,909, filed August 29,19-56, referring particularly to FIGS. 3 and 7, showing rotatable phaseshifting delta sections.

The side slot coupler 23 may be a coupler of the Riblett type.

Signals arriving at the antenna from directly on axis "two feed horns 19and 20. However, due to the presence of the fixed phase shift 22 in oneof the feed horn waveguides, the inputs to the side slot coupler 23 inthe feed section will be equal in amplitude but in phase quadrature.Owing to the inherent 90 phase shift of signals coupling through theside slot coupler, all of the energy will be transmitted into the rangechannel 47.

Deviation from axis in elevation will result in an excitation of the twohorns 19 and 20 with signals of unequal amplitude and negligible phasedifference due to the opposite offsets of the antenna patterns of eachfeed horn. Since the signals now impressed on the side slot coupler areunequal in amplitude, a component will result in the error channel themagnitude of which is proportional to the amount of deviation oif axisin elevation. The sense of this deviation may be obtained by acomparison of the phase of the elevation error signal with the rangechannel signal.

It will be observed that in the azimuth plane, the feed waveguides 15and 1 6 shield each half of the parabolic reflector 11 from oppositehorn illumination. This configuration results in two antenna patternsthe amplitude distributions of which are almost identical but havingphase centers displaced by approximately half the width of the parabolaas illustrated by the points 49 and 50 of the schematic diagram of FIG.7. For deviations from the axis in the azimuth plane, signals areexcited at the two feed horns which are essentially equal in amplitudebut differ in phase, depending upon the amount of azimuth deviation.Since the signals are now impressed on the side slot coupler equally inamplitude but at some relative phase angle, a signal will again resultin the error track channel 48. The amount of this signal will beproportional to the amount of azimuth deviation and the sense of theazimuth deviation can again be obtained by comparison of the errorchannel and range channel phases.

The manner of indication of relative phase is illustrated by thediagrams of FIGS. 4 and 5. Fig. 4 illustrates the condition when the twohorns of the feed are excited unequally but in phase. The inputs to theside slot coupler 23 and the range and elevation channel components areas shown. The vectors representing the energy in waveguides 16 and 15are designated by the reference characters E and E Since B is eitherlarger or smaller than E depending upon the direction of the elevationerror, the resultant elevation error signal B is either leading orlagging the range signal E by 90. E, is the resultant of E component ofE, continuing through the range channel 15, and E the component of E,transferred through the side slot coupler 23 to the range channel 15. Bis likewise the resultant of E the component of E transferred throughthe side slot coupler 23 to the error channel 16, and E the component ofB continuing through the error channel 16.

Referring to FIG. 5, the inputs to the feed and side slot couplers forazimuth deviation from the axis are shown. This also results incomponents in the range and error channels as indicated. Depending uponthe sign of the phase angle 0, the azimuth error signal is either inphase or out of phase with the range channel signal; hence the azimuthand elevation error signal are in phase quadrature in the same errorchannel This permits ready separation of azimuth and elevation errorsignals in the receiver as readily understood by those skilled in theart.

Consequently only two receivers are required.

In FIG. 5, E,,I and E I are the vectors representing input to thewaveguides 16 and .15, respectively, when the phase angle 0 is positive;E,,I I and E -II are the corresponding vectors when the phase angle 0 isnegative.

E -I and E -I are the components supplied to the range channel, and E -Iand E -I the components supplied to the elevation channel, for positivephase angle 0. E,.I and E,I are the resultants for positive value of 0.

For negative values of the phase angle 0, the cone sponding vectors areE -II and E -II, E -II and E -II and the resultants E -II and E -II.Whether the phase angle!) is positive ornegative is revealed by whetherthe resultantin the error channel for azimuth error is in or out ofphase with the resultant in the range channel.

The arrangement illustrated avoids the mechanical complexity andnumerous moving parts of physically rotating reflectors or feedarrangements. There is only the simple mechanical rotation involved inthe Fox type phase shifter 46. The simplified microwave assemblyillustrated therefore, accomplishes monopulse operation with reducedcomplexity of fabrication and maintenance, improved performance andgreater accessibility of components.

In order that a constant intermediate frequency will be obtained for usein the receivers connected to the range and error mixers 29 and 30, anautomatic frequency control mixer 51 may be provided having inputchannels 52 and 53 from the local oscillator 28 and the automaticfrequency control coupler 37, respectively, with an output channel 54connected to suitable means as will be understood by those skilled inthe art for adjusting the frequency of the local oscillator 28 tocorrespond to any possible fluctuation in frequency of the magnetron Thedual TR tubes 24 and 26 preferably have built-in shutters for crystalprotection. The arrangement permits mounting the local oscillator 28together with the duplexers including the TR tubes 24 and 26, the sideslot couplers 25 and 27, 31 and 32 and the mixers 29 and 30 upon thescanner assembly including the reflector 11.

The conical scanning of the transmitted beam takes place at a ratedetermined by the change of phase shift in the channel 43 produced bythe Fox type phase shifter 46. The arrangement illustrated allows fortransmission of such a conical scanning beam while retaining theinherent accuracy of monopulse tracking.

Referring to FIG. 9, the full line curve 56 represents the range signalenergy beam in one position and the dotted line curve 57 represents aposition of this conically scanning beam 180 from that represented bycurve 56.

In FIG. the dash line curves 58 and 59 represent for either the azimuthplane or the elevation plane relative response in the error channel tosignals or reflections plotted according to angles from the axis of thereflector 11. The full line curve 60 represents the response in therange channel to reflections from the located object plotted accordingto deviation angle of the object from the axis of the reflector L1.

Although the invention has been described and illustrated in detail, itis to be clearly understood that the same is by way of illustration andexample only and is not to be taken by way of limitation, the spirit andscope of this invention being limited only by the terms of the appendedclaims.

We claim:

1. A radiant energy system comprising in combination a reflector, a twohorn feed therefor, a transmitter, a pair of output channels coupled tosaid transmitter, a side slot coupler coupling the two horn feed to saidoutput channels with 90 phase shift interposed in one of the channels,duplexers interposed in each of the channels, range receiver output forone of the duplexers, error receiver output for the other duplexer, anda continuous phase shifter interposed in one of said transmitterchannels.

2. A radiant energy system comprising in combination a power oscillator,a local oscillator, a mixer responsive to said oscillators, said localoscillator having frequency control, said mixer having output responsiveto said local oscillator for adjusting frequency to maintain apredetermined relationship between power oscillator and local oscillatorfrequencies, a power splitter connected to said power oscillator toreceive energy thereof, a pair of chan- 6 nels connected to said powersplitter, one of said channels having a continuously variable phaseshifter interposed therein, a reflector, a two horn feed, a side slotcoupler coupling said channels to said two horn feed, one of saidchannels having phase shift interposed therein, said two horn feed beingmounted asymmetrically with respect to the reflector, and duplexersinterposed in the channels with separate receiver outputs, wherebyconical scan is obtained with respect to transmission and monopulse scanwith respect to reception from said reflector.

3. A reflector, first means for feeding energy to the reflector, secondmeans for feeding energy to the reflector with a diiferent phase, thecenters of said first and second means being located on a line whichforms a substantially 45 degree angle with predetermined mutuallyperpendicular azimuth and elevation axes, and a continuous phase shifterinterposed in one of said means for effecting conical scan.

4. The radiant energy system comprising in combination a poweroscillator, a local oscillator, a mixer responsive to said oscillators,said local oscillator having frequency control, said mixer having outputresponsive to said local oscillator for adjusting oscillator frequencyto maintain a predetermined relationship between power oscillator andlocal oscillator frequencies, a pair of power channels connected to saidpower oscillator, a reflector with a two-horn feed and a side slotcoupler coupling said power channels to said two-horn feed, saidtwo-horn feed being mounted asymmetrically with respect to thereflector, duplexers interposed in the channels with separate receiveroutputs whereby range and tracking error signals may be obtained fromthe reflector.

5. A radiant energy system comprising in combination a reflector, a twohorn feed device therefor, the centers of each horn of said feed devicebeing located on a line which forms an angle of substantially 45 degreeswith the azimuth and elevation axes of said system, a transmitter, apair of output channels coupling the transmitter to the reflector feeddevice, one of said channels having a fixed delay therein, and acontinuously variable phaseshifter being interposed in one of saidchannels.

6. A radiant energy system comprising in combination a reflector, atwo-horn feed device therefor having a hybrid bridge therein, atransmitter, a pair of output channels coupling the transmitter to thereflector feed device and a continuously variable phase shifterinterposed in one of said channels.

7. A radiant energy system comprising in combination a reflector, a dualfeed device therefor having a pair of displaced phase centers, atransmitter, a pair of output channels coupling the transmitter to thereflector feed device and a continuously variable phase shifterinterposed in one of said channels.

8. A radiant energy system comprising in combination a transmitter,first and second channels coupled to said transmitter, a continuousphase shifter interposed in one of said channels, a side slot couplerhaving input ports connected to said first and second channels, a pairof waveguide sections each separately connected to said side slotcoupler, one of said waveguide sections having a 90 phase shift ascompared with the other of said sections, a pair of feed horns, each ofsaid horns connected to a separate one of said waveguide sections, and areflector, each of said horns being positioned to illuminate a separatehalf of said reflector.

9. A radiant energy system comprising in combination a transmitter,first and second channels coupled to said transmitter, one of saidchannels having a continuous phase shifter interposed therein, a sideslot coupler connected to receive the outputs of said channels, areflector, and two horn feed means connected to said side slot couplerfor coupling the output of said coupler to said reflector with a 90phase shift in one of said feed means as compared with the other of saidfeed means, each of said feed horn means being positioned to illuminatea separate half of said reflector.

10. In a radiant energy system in combination, a refleeshift meansinterposed in one of said channels for eifecttor, a pair of feed hornsmounted in the vicinity of the lug conical scan. focal point of saidreflector, the centers of said horns being located on a line which formsa substantially 45- References cued the file of thls Patent degree anglewith both the elevation and azimuth axes of 5 UNITED STATES PATENTS thesystem, a pair of waveguide channels coupled to said 2,513,498 LawsonJuly 14, 50 pair of feed horns, one of said channels having an efieo- 7,403 Marsh et 1 5, 1957 tively one-fourth wavelength longer transmissionpath 2331733 5 W thi t D 24, 1957 than the other of said channels, andcontinuous phase 2,824,305 Ohlemacher Feb. 18, 1958

